1. Field of the Invention
This invention relates to ungrounded or isolated power supplies. In particular, it relates to surveilance means for an installation comprising several isolated power supplies.
2. Description of the Prior Art
The need for ground fault detection in electric power distribution is as old as the power distribution art. Many devices have been proposed to aid the power supplier in sensing and finding such faults. In recent years, hospitals have found it desirable to use isolated power supplies for use in operation rooms, etc., so as to avoid as much as possible accidents resulting from electrical sparks, shocks, etc. To indicate that the isolated power supplies are actually free of grounds, line isolation monitors have been developed. A regulatory agency, National Fire Prevention Association (NFPA - 56, 1956), has set up standards for ground isolation in hospitals. To comply with the standards, a total leakage no greater than 0.002 amperes from line to ground is allowable. Included in this leakage figure is the capacitative leakage of the power supply itself, and the power required to drive the ground indicating device. With the limits on leakage as set up by NFPA, it is desirable that the driving power of the ground sensor be kept at as low a value as possible so that the growth of leakages below the allowable maximum can be watched. In general, the leakage contribution of many presently known ground fault detectors is of the order of 0.0005 amperes. With the limit of 0.002 amps total leakage for a system, the relatively high current draw of such devices prevents any accurate reading of the possible fault currents.
In a typical hospital installation, power is fed from the utilities lines to an isolation transformer. Non-grounded power lines from the isolation transformer feed one or more operating rooms, intensive care wards, etc. A line isolation monitor is connected to the non-grounded lines at any suitable spot. In a preferred installation, a signal is provided at each use area to advise that the power line is either: (a) free from grounds or (b) that a ground fault exists. In many hospital installations, there may be several isolated power supply systems, each with its own monitor. It is further requisite of NFPA-56 that each isolated power supply must be checked daily for leakage. In a large installation, say of ten or more separate power supplies, the daily check by trained personnel may require a considerable outlay of time in the course of a year for example. A means for electrically testing a number of isolated power supplies from a central point could be of real benefit to such an installation.